The invention relates to a method and an apparatus for ultrasound testing of a disc body which is shrunk onto a shaft, in particular a disc of a low-pressure turbine rotor.
Stress-corrosion cracks can lead to progressive damage when rotating parts are subject to dynamic loading, such as a disc on a rotor of a low-pressure turbine, for example. It is thus necessary to examine such components at predetermined intervals for stress corrosion cracks. Discs which are arranged on a rotor such that they are locked by axial or radial pins against rotation cannot be tested by dismantling the rotor, for economic reasons. It is thus recommended that an ultrasound test has to be carried out in the region of the shrink seating and of the adjacent material regions, in particular in the region of the hub inner surface and the axial anti-rotation locks. However, the geometry of the disc is extremely complicated for ultrasound testing using ultrasound test heads in the inaccessible hub inner region and in the vicinity of the anti-rotation locks. Furthermore, additional problems are associated with carrying out the ultrasound tests in a reproducible manner. Only the side pieces of the disc are available as a coupling surface for the ultrasound test heads. Accessibility to the coupling surfaces on the disc is frequently restricted by adjacent discs. A further problem is that the sound paths to reflectors governed by the geometry (for example an anti-rotation lock) and to any cracks which may be present originating from the hub bore may be identical.
In order to overcome these difficulties, EP 0 126 383 B1 has proposed a method for ultrasound testing of disc bodies, which are shrunk onto shafts, in the region of the shrink seating, and a device for carrying out the method. According to this proposal, special angled test heads are arranged together with measuring probes on a metal template sheet, which is matched to the respective side piece section, with the aid of a test head holder. Such special angled test heads are then moved to, and coupled to, the contour of the section of the relevant side piece by means of a manipulator arm. A specific subregion of the disc is assigned a metal template sheet. The correct axial and radial positioning of the metal template sheet is indicated by test probes. In other words, the measuring probes, which are designed in particular as inductive measuring probes, are arranged on each metal template sheet in order to monitor the positioning of the angled test heads, and the test heads themselves are in each case arranged at quite specific positions, and with quite specific relationships to one another, in order to ensure the conversion of the ensonification and squint angles of the relevant test heads to the ensonification directions required for the specific test region, when coupling to a specific position on a disc side piece. This results in a multiplicity of metal template sheets being necessary for testing a disc. The manufacture of such metal template sheets is dependent on the geometry of the side pieces being known. The preparatory work which is required to carry out the ultrasound test is relatively complex, since ultrasound testing is not possible until after comprehensive, time-consuming analysis of the drawings and after design and production of manipulation aids in the form of templates.
The brief extract from Japanese Patent Application JP-A 63-309 852, from Patents Abstracts of Japan P-854 Apr. 11, 1989, Vol. 13/No. 146 describes a method for crack identification in a body having an essentially unknown contour which is curved in a continuous convex manner. The contour is determined using a scanning device which can be moved in three mutually independent directions, is stored in a memory, and these stored data are used to control the drive mechanism of a manipulator. A distance sensor is mounted on the scanning device. The data stored in the memory are used in a computer to calculate the position of a test probe, so that the latter is guided essentially at right angles to the surface of the body, at a predetermined distance from it. A further method for determining the contour of a body and for identification of cracks in the body is described in GB 2 266 367 A. This method is carried out for determining an inner and an outer contour of a three-dimensional test object as well as for finding deeply located defects in metallic components, for example turbine blades, pipelines or valves. The contour of the test object is determined using an ultrasound test head coupled to it, in such a manner that the respective position of the ultrasound test head is detected via LED probes.